1. Field of the Invention
The present invention relates to a location system for locating a cooperative beacon. More particularly, it concerns a location system based on Global Positioning System ("GPS") technology that permits accurate range and direction detection of a cooperative beacon using relative GPS ("RGPS") differential range and direction.
2. Description of Related Art
The Global Positioning System ("GPS") presently consists of a constellation of 24 satellites that continuously broadcast time and frequency data on two frequencies, L1 and L2. GPS receivers have been developed that can determine a precise location on the earth by measuring the time required for the signals of three, or preferably four or more, of the satellites to reach the location on earth. Using these measurements and triangulating, the GPS receiver's position may be calculated. The GPS receiver provides position information in the form of X, Y, and Z axes coordinates measured in meters from the center of the earth. The position information is provided using 9 significant digits (e.g., 4137958.36).
GPS provides both a commercial version of GPS data and secure versions of GPS data. The secure versions of the GPS data requires the use of classified codes provided by the United States Government to decode the data. The commercial version of GPS data, referred to as C/A ("Coarse Acquisition") code GPS, uses only the L1 frequency and does not require any decoding. However, the United States Government imposes pseudo-random errors (referred to as selective availability ("S/A") random errors) into the location data provided by the satellites. Therefore, due to this introduced error, existing C/A code GPS based navigation aids provide a low resolution direction to a fixed point on the earth, i.e. a waypoint, with only an approximation range to the desired location. The Department of Defense controls the S/A random errors and determines the true accuracy of C/A commercial user GPS. The accuracy is nominally set to 300 meters spherical (3 d RMS) or 100 meters horizontal (2 d RMS). The classified version requiring code settings (Y-code) is much more accurate (on the order of 16 meters).
Alternatively, prior art locator systems using C/A mode GPS provide more accuracy by using differential GPS ("DGPS") techniques. These existing C/A mode DGPS based systems require a surveyed base site capable of calculating the introduced error from the C/A mode GPS data. The surveyed base site transmits the calculated error information to the locator. From its own C/A mode GPS data and the calculated error transmitted from the surveyed base site, the locator can correct for any introduced S/A error.
Existing GPS based locator systems further define their bearing information based on movement along a baseline. Until the user moves, however, there is no way for the user to determine the direction that they are going because GPS provides only an instantaneous position. Therefore, two subsequent positions are required to define direction.
Thus, a need has arisen for a system capable of accurately locating a beacon from a locator without using any external aids or surveys. In addition, a need has arisen for a locator in which no motion is required to determine range or bearing information. Further, a need has arisen for a cooperative location system operating at low (truncated) data rates in order to permit efficient transmission of information between a beacon and a locator.